A standard vibratory pile driver has a heavy housing provided with at least two shafts carrying eccentric weights. The shafts are rotated at high speed to vibrate the housing which is clamped to the upper end of the pile to be driven. This vibration, combined with the weight of the driver, causes the pile to sink into the ground or bottom, or conversely the housing can be pulled upward so the vibration loosens the pile and it can be pulled out. Typically the housing is suspended by means of an elastomeric vibration damper from the cable of a crane so that the vertical vibration is transmitted to the pile, not back up the cable to the crane.
Such an arrangement is used to drive sheet piling for forming bulkheads below ground, for canals, or around foundations. Hydraulic power is preferred because it eliminates the danger of electricity when working around water and allows a stepless adjustment of vibration rate. In addition hydraulic motors can be made very compact even when relatively powerful.
It has always been considered necessary to link the various shafts together. This is typically done by providing continuous-mesh gearing between them so that the rotation of the drive motor is positively transmitted to one shaft and thence via the gear train to the other shaft or shafts. Such gearing obviously wastes power and requires lubrication and frequent servicing. The high peripheral speed of such gearing is often near the limit it is rated for. Normal gears are rated at a peripheral speed of at most 18 m/sec, whereas in a typical vibratory hammer the speed can approach 30 m/sec, putting an excessive strain on the gears. As a result the gearing interconnecting the shafts of the pile driver usually has an excessively short service life. The high speed further heats the lubricating oil for the gearing, requiring special construction to dissipate and/or withstand this heat.
Another problem with these systems is that the gearing generates a great deal of noise. Whereas the hydraulic motor of the driver can operate almost perfectly silently, the gearing can produce a high-pitched whine that is close to the maximum tolerable sound limit. Workers closely exposed to the driver must wear ear protectors, and the noise level increases as the gears wear.
Another problem with the known driver is that it is very heavy and, due to accommodating the above-described gearing inside its housing, is relatively tall. Thus the center of gravity of the driver lies well above the top of the pile to which it is clamped. If insufficient tension is maintained on the cable from which the driver is suspended, it can tip to the side and fall or at least damage the pile being driven.
Finally the ability of the known pile driver to pull a pile is often considerably limited by the vibration damper that is provided as part of the housing, which damper further increases the height of the unit. In order to effectively damp the vibrations of the machine and protect the crane from them, heavy coil-spring or block-type damping units are provided which can only transmit limited tension. If therefore becomes necessary to exert a relatively low tension when extracting a pile, or the damper is damaged.